By definition, keratoconus is a non-inflammatory, usually bilateral protrusion of the cornea, the apex being displaced downward and nasally. Essentially, keratoconus is caused by a thinning of the cornea that will typically result in asymmetric astigmatism. Another aspect of the disease, however, also deserves consideration. Specifically, this consideration is the fact that keratoconus causes a loss of corneal tissue that may effectively preclude an individual from pursuing particular refractive surgical procedures for vision correction. For example, due to the inherent loss of corneal tissue, keratoconus would be contraindicative for procedures that involve the removal of corneal tissue (e.g. the well-known LASIK surgery).
In the early stages of the disease, keratoconus is not easily detected. Specifically, the pronounced change in the shape of the cornea that is characteristic of advanced keratoconus, is not noticeably evident in the early stages. Nevertheless, there are structural weaknesses in the corneal tissue, caused by lamella crossover in the early stages of the disease, that portend the disease. It is extremely difficult, however, if not impossible to locate and directly measure the biomechanical stresses and strains that are characteristic of these weaknesses.
Anatomically, the cornea of an eye comprises five identifiable layers of tissue. In an anterior-posterior direction these layers are: epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium. The stroma forms about 90% of the corneal thickness, with Bowman's membrane forming most of the remaining thickness. Though smaller than the stroma, Bowman's membrane is around five times stronger and is more elastic than is stromal tissue. The remaining layers (i.e. epithelium, Descemet's membrane, and endothelium) provide negligible structural strength for the cornea. Accordingly, based on layer thickness and relative structural considerations (i.e. biomechanical parameters) for Bowman's membrane and the stroma, a mathematical model of a cornea can be established using well known mathematical techniques.
Unlike the difficulties mentioned above that are encountered in measuring biomechanical characteristics of corneal tissue, surfaces of the cornea can be more easily defined. In particular, respective topographies for the anterior and posterior surfaces of a cornea can be obtained using known imaging techniques (e.g. second harmonic generation imaging). Further, it is known that the shape of a cornea, as determined by its surface topographies, is a consequence of the stress-strain relationships experienced by tissues inside the cornea.
In light of the above, it is an object of the present invention to provide a system and method for diagnosing a keratoconic cornea at the onset of the disease, prior to any observably noticeable change in the anatomical shape of the cornea. Another object of the present invention is to provide a system and method for determining whether a cornea is a proper candidate for subsequent refractive surgical procedures. Yet another object of the present invention is to provide a system and method for diagnosing a keratoconic cornea that is simple to employ, is easy to use and is comparatively cost effective.